When the device is used for the first time, the dose of a drug actually delivered will be less than the dose set, and the difference will equal the volume that would be ejected if the bung were travelling at the speed of the piston rod before the piston rod gets into contact with the bung. For many drugs this loss is significant and can cause the first dose to be well outside the allowable dose accuracy limits. The user is therefore instructed to perform a priming step like ‘air shot’ prime doses until fluid begins to be ejected.